This invention is related to a pigment dispersant this is useful in a cathodic electrocoating composition.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process an autobody or an auto part, is immersed in a bath of an electrocoating composition containing an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counterelectrode in electrical contact with the aqueous emulsion until a desired thickness of coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from a polyepoxide which has been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coatings that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at a sufficient temperature to produce a crosslinked finish on the article.
Cathodic electrocoating compositions, resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al. U.S. Pat No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat No. 4,137,140 issued Jan. 30, 1979 and Wismer et al. U.S. Pat No. 4,468,307 issued Aug. 25, 1984.
Pigments are a necessary component in a typical electrocoating automotive primer composition. Pigment dispersants are used to disperse the pigment in the composition and keep the pigment dispersed in the composition and thus are a very important part of any electrocoating composition. Useful pigment dispersants for cathodic electrocoating compositions are disclosed in Gebregiorgis et al. U.S. Pat. No. 5,128,393 issued Jul. 7, 1992, Gebregiorgis et al. U.S. Pat. No. 4,946,507 issued Aug. 7, 1990 and Gebregiorgis U.S. Pat. No. 5,116,903 issued May 26, 1992.
In the process for forming pigment dispersions for cathodic electrocoating compositions, primary pigment particles are separated from agglomerates or aggregates of these particles; accluded air and absorbed water are displaced and the surface of the pigment is coated with the pigment dispersant. Ideally, each primary particle which has been mechanically separated during the dispersion process, also is stabilized against flocculation. If the pigment particles are not properly dispersed and stabilized in the composition, the advantages built into the pigment by the manufacturer may be lost. For example, the pigment may settle in the electrodeposition bath which can result in loss of corrosion protection of the substrate. In addition, appearance of a film deposited by the electrodeposition process and the operating characteristics of an electrocoating bath may be adversely affected by inadequate pigment dispersion. The better the pigment dispersant used in a coating composition or electrocoating bath, the less dispersant is required and the pigment to binder ratio can be increased in the composition. This can result in a savings on dispersant costs, improved processability, and a lower VOC (Volatile Organic Content) of the electrocoating bath.